Session J1: The Gap Structure of the Fe Superconducters

Spin fluctuation pairing in Fe-based superconductors and its consequences

The new Fe-based superconductors have occasioned excitement because transition temperatures are high and it is hoped that comparisons to cuprates can lead to new insights on the essential ingredients to high temperature superconductivity. According to conventional weak coupling spin fluctuation models, $A_{1g}$ (sign-changing ``s-wave'') states are probably favored. Such states may be isotropic on each Fermi surface sheet or highly anisotropic, possibly with order parameter nodes. I discuss how the anisotropy of the ground state can depend on interaction parameters, electronic structure, and disorder effects. Experiments indicating a possible gapped-nodal crossover in these systems will be discussed in this framework.\\[4pt] [1] S Graser et al., New J. Phys. \textbf{11}, 025016 (2009).\\[0pt] [2] T. A. Maier et al., Phys. Rev. B \textbf{79}, 224510 (2009).\\[0pt] [3] K. Kuroki, H. Usui, S. Onari, R. Arita, and H. Aoki, Phys. Rev. B \textbf{79}, 224511 (2009)   

Non-Fermi liquid behavior and non-universal superconducting gap structure in Fe-pnictides

The discovery of Fe-pnictide superconductors with $T_c$ exceeding 55~K raises fundamental questions about origin of high-$T_c$ superconductivity. Here we report the systematic studies of the normal-state charge transport, Fermi surface structure and superconducting gap structure in high-quality single crystals of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ ($0 \leq x \leq 0.71$), ranging from the SDW state to overdoped Fermi liquid state. Near the SDW boundary, the transport coefficients, including resistivity, Hall coefficient and magnetoresistance, exhibit striking deviations from the Fermi liquid properties [1]. The Fermi surface structure determined by the dHvA effect shows that in the superconducting dome the volume of the electron and hole sheets shrink linearly and the effective masses become strongly enhanced with decreasing $x$ [2]. It is likely that these trends originate from the many-body interaction which gives rise to superconductivity. The penetration depth, thermal conductivity and NMR data for BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ ($T_c$=30~K) provide unambiguous evidence for line nodes in the superconducting gap function [3], in sharp contrast to the other Fe-based compounds with fully gapped structure. This indicates that the gap structure of Fe-based high-$T_c$ superconductors is not universal.\\ \\ \noindent [1] S. Kasahara {\it et al.}, arXiv:0905.4427 [2] H. Shishido {\it et al.}, arXiv:0910.3634 [3] K. Hashimoto {\it et al.}, arXiv:0907.4399 [4] K. Hashimoto {\it et al.}, Phys. Rev. Lett. {\bf 102}, 017002 (2009), {\it ibid} {\bf 102}, 207001 (2009).   

Anisotropic London Penetration Depth in Iron-based Pnictide Superconductors

The temperature dependent London penetration depth, $\lambda \left( T \right)$, is linked directly to the structure of the superconducting gap, thus providing valuable insight into the pairing mechanism. I will summarize measurements of the penetration depth in single crystals of iron-based pnictide superconductors comparing the ``1111'', ``11'' and ``122'' families. Compatibility of our results with other gap sensitive probes, such as ARPES and thermal conductivity, will be addressed. A detailed discussion of the doping dependent penetration depth will be given for the well-characterized ``122'' family, (Ba$_{1-y}$K$_{y})$(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni, Pd, Rh). Overall, $\lambda \left( T \right)$ exhibits a power law variation at low temperatures, $\lambda \left( T \right)=\lambda \left( 0 \right)+bT^n$ (down to 80 mK in the case of FeNi-122). The exponent $n$ is typically less than 2.8, which is clearly different from $n\approx 4$ that parameterizes the exponential behavior expected for conventional fully gapped s-wave superconductors. The low-temperature parameters, $\lambda \left( 0 \right)$, $b$ and $n$ depend on the doping level and the orientation of a magnetic field with respect to the crystal axes. This evolution is best observed in the out-of-plane penetration depths, $\lambda _c \left( T \right)$, which at least in the FeNi-122 system, changes from a high power in the underdoped regime to $T-$linear in overdoped samples. Simultaneously, the in-plane penetration depth, $\lambda _{ab} \left( T \right)$, evolves towards a sub-quadratic behavior with $n\approx 1.7$. Furthermore, analysis of the superfluid density in the full temperature range is consistent with two-gap superconductivity. However, the temperature dependencies of the anisotropies, ${\lambda _c } \mathord{\left/ {\vphantom {{\lambda _c } {\lambda _{ab} }}} \right. \kern-\nulldelimiterspace} {\lambda _{ab} }$ and ${\xi _{ab} } \mathord{\left/ {\vphantom {{\xi _{ab} } {\xi _c }}} \right. \kern-\nulldelimiterspace} {\xi _c }$, are opposite compared to another two-gap superconductor, MgB$_{2}$. Consistency of these results with theories that explain the power law behavior to be due to scattering in a two-dimensional $s_\pm $ model will be discussed. Overall, our results suggest that the superconducting gap in iron-based pnictide superconductors develops nodal structure in the overdoped regime with nodes located at finite $k_z $ wave vectors on a three-dimensional Fermi surface. \\[4pt] \textbf{References:} C. Martin \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 247002 (2009); R. T. Gordon \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 127004 (2009); R. T. Gordon \textit{et al.}, Phys. Rev. B \textbf{79}, 100506(R) (2009)   

Ferropnictides at high magnetic fields: the role of pairing symmetry and impurity scattering

An overview of recent results on the effect of impurity scattering and pairing symmetry on the upper critical field $H_{c2}(T)$ and critical temperature of oxypnictides at very high magnetic fields is given. The role of multiband effects and different scattering impurity channels on the observed anomalous temperature dependencies of anisotropic $H_{c2}(T)$ significantly exceeding the BCS paramagnetic limit is addressed.   

Gap structure of iron-pnictide superconductors from low-temperature heat transport

The structure of the superconducting gap provides important clues on the symmetry of the order parameter and the pairing mechanism. Here I describe how measurements of the thermal conductivity at very low temperature can be used to determine whether nodes are present in the gap function of a particular superconductor, and how the application of a magnetic field probes the low-energy quasiparticle excitations. Measurements on hole-doped and electron-doped pnictide superconductors, Ba$_{1-x}$K$_x$Fe$_2$As$_2$ [1] and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ [2], reveal a negligible residual linear term at T$\to$0, showing that the gap of these two superconductors has no nodes, at least in the basal plane. In both pnictides, a small field is found to be very effective in exciting quasiparticles, showing that the gap must be very small in some direction on the Fermi surface. In Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, the evolution with doping x is as follows: at low x, the gap is large everywhere on the Fermi surface, and beyond optimal doping the minimum gap becomes progressively smaller. I discuss what these features tell us about the nature of the superconducting state in pnictide superconductors. * Measurements of heat transport performed in collaboration with X.-G. Luo, H. Shakeripour, M.A. Tanatar, N. Doiron-Leyraud and L. Taillefer. [1] X.-G. Luo et al., Phys. Rev. B 80, 140503 (2009). [2] M.A. Tanatar et al., arXiv:0907.1276.